Quantum capacitance graphene varactors and fabrication methods

ABSTRACT

A plate varactor includes a dielectric substrate and a first electrode embedded in a surface of the substrate. A capacitor dielectric layer is disposed over the first electrode, and a layer of graphene is formed over the dielectric layer to contribute a quantum capacitance component to the dielectric layer. An upper electrode is formed on the layer of graphene. Other embodiments and methods for fabrication are also included.

BACKGROUND

Technical Field

The present invention relates to devices with variable capacitance(varactors), and more particularly to varactors formed with graphenedielectric.

Description of the Related Art

Varactors in silicon technologies have a maximum C_(max)/C_(min) ratioof 5. This limits the performance of circuits that require variablecapacitance, e.g., voltage-controlled oscillators (VCOs). A varactor isan essential component of multiple digital, analog and mixed-signalintegrated circuits (ICs). At mmWave frequencies latencies betweenpassive and active components must be minimized. This typically requiresclosely integrated components (preferably in the same substrate) foroptimum performance.

SUMMARY

A plate varactor includes a dielectric substrate and a first electrodeembedded in a surface of the substrate. A capacitor dielectric layer isdisposed over the first electrode, and a layer of graphene is disposedover the first electrode in contact with the dielectric layer tocontribute a quantum capacitance component to the dielectric layer. Anupper electrode is formed on the layer of graphene.

Another plate varactor includes a dielectric substrate and a layer ofgraphene formed over a portion of a surface of the substrate. A firstelectrode is formed contacting edges of a periphery of the layer ofgraphene and over portions of the surface of the substrate. A dielectriclayer is formed over the layer of graphene and over a part of the firstelectrode contacting the layer of graphene. The dielectric layer and thelayer of graphene provide a capacitor dielectric wherein the layer ofgraphene contributes a quantum capacitance component to the dielectriclayer. An upper electrode is formed on the dielectric layer.

A method for fabricating a plate varactor includes forming a trench in adielectric substrate; forming an embedded electrode in the trench of thesubstrate; forming a capacitor dielectric layer over the firstelectrode; providing a layer of graphene over the first electrode tocontribute a quantum capacitance component to the dielectric layer; andforming an upper electrode on the layer of graphene.

Another method for fabricating a plate varactor includes providing alayer of graphene formed over a portion of a surface of a dielectricsubstrate; forming a first electrode contacting edges of a periphery ofthe layer of graphene and over portions of the surface of the substrate;forming a dielectric layer over the layer of graphene and over a part ofthe first electrode contacting the layer graphene, the dielectric layerand the layer of graphene providing a capacitor dielectric wherein thelayer of graphene contributes a quantum capacitance component to thedielectric layer; and forming an upper electrode on the dielectriclayer. Other embodiments and methods for fabrication are also included.

These and other features and advantages will become apparent from thefollowing detailed description of illustrative embodiments thereof,which is to be read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

The disclosure will provide details in the following description ofpreferred embodiments with reference to the following figures wherein:

FIG. 1 is a cross-sectional view of a varactor with a capacitordielectric including graphene enclosed between two electrodes inaccordance with one embodiment;

FIG. 2 is a cross-sectional view of a dielectric substrate provided inaccordance with one illustrative embodiment;

FIG. 3 is a cross-sectional view showing a trench formed in thedielectric substrate of FIG. 2 in accordance with one illustrativeembodiment;

FIG. 4 is a cross-sectional view of an embedded electrode formed in thedielectric substrate of FIG. 3 in accordance with one illustrativeembodiment;

FIG. 5 is a cross-sectional view showing a dielectric layer formed onthe embedded electrode of FIG. 4 in accordance with one illustrativeembodiment;

FIG. 6 is a cross-sectional view showing a graphene layer patterned onthe dielectric layer of FIG. 5 to correspond to the embedded electrodein accordance with one illustrative embodiment;

FIG. 7 is a cross-sectional view showing an upper electrode formed overthe graphene layer of FIG. 6 and corresponding to the embedded electrodein accordance with one illustrative embodiment;

FIG. 8 is a cross-sectional view of a varactor with a capacitordielectric including graphene sandwiched between two different sizedelectrodes in accordance with another embodiment;

FIG. 9 is a cross-sectional view of a dielectric substrate provided inaccordance with one illustrative embodiment;

FIG. 10 is a cross-sectional view showing a graphene layer formed on thedielectric substrate of FIG. 9 in accordance with one illustrativeembodiment;

FIG. 11 is a cross-sectional view showing the graphene layer of FIG. 10being patterned in accordance with one illustrative embodiment;

FIG. 12 is a cross-sectional view showing a first electrode faulted overa peripheral region (edges) of the graphene layer of FIG. 11 inaccordance with one illustrative embodiment;

FIG. 13 is a cross-sectional view showing a dielectric layer formed andpatterned on the first electrode of FIG. 12 in accordance with oneillustrative embodiment;

FIG. 14 is a cross-sectional view showing an upper electrode formed onthe dielectric layer and over the graphene layer of FIG. 13 inaccordance with one illustrative embodiment;

FIG. 15 shows three plots of capacitance versus applied voltage to showthe tunability of varactors in accordance with the present principles;

FIG. 16 is a block/flow diagram showing steps for fabricating a varactorof FIG. 1 in accordance with illustrative embodiments;

FIG. 17 is a block/flow diagram showing steps for fabricating a varactorof FIG. 8 in accordance with illustrative embodiments; and

FIG. 18 is a cross-sectional view of a varactor with a capacitordielectric including graphene sandwiched between two different sizedelectrodes in accordance with another embodiment.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In accordance with the present principles, graphene varactors areprovided. The varactors include a layer of graphene disposed between twoelectrodes. In one embodiment, one of the electrodes may be embedded ina top portion of a substrate. High dielectric constant (high-K) materialmay be formed over the embedded electrode, with a layer of graphene overthe high-K dielectric material. An upper electrode may be formed on thelayer of graphene. In this case, both electrodes completely cover thegraphene area, minimizing the contact resistance and improving thevaractor's quality factor.

In another embodiment, a graphene varactor is formed having a layer ofgraphene formed over a portion of a surface of the substrate. Oneelectrode is formed to contact edge regions of the graphene and overother portions of the surface of the substrate. A dielectric layer(preferably high-K) is formed over the graphene and the part of theelectrode contacting the graphene. An upper electrode is then formed onthe dielectric layer.

In accordance with the present principles, plate capacitor typevaractors are provided, which preferably include a single layer ofgraphene between electrodes. No gate or diffusion regions are needed asthe present principles do not employ field effect transistor typevaractors. The present principles instead provide plate varactorscompatible with graphene process technology, which employ quantumcapacitance effects of the graphene material to adjust capacitance ofthe varactor. Such varactors provide improvements needed for futuregenerations of integrated circuits.

It is to be understood that the present invention will be described interms of a given illustrative architecture having a wafer or substrate;however, other architectures, structures, substrate materials andprocess features and steps may be varied within the scope of the presentinvention.

It will also be understood that when an element such as a layer, regionor substrate is referred to as being “on” or “over” another element, itcan be directly on the other element or intervening elements may also bepresent. In contrast, when an element is referred to as being “directlyon” or “directly over” another element, there are no interveningelements present. It will also be understood that when an element isreferred to as being “connected” or “coupled” to another element, it canbe directly connected or coupled to the other element or interveningelements may be present. In contrast, when an element is referred to asbeing “directly connected” or “directly coupled” to another element,there are no intervening elements present.

A design for an integrated circuit chip may be created in a graphicalcomputer programming language, and stored in a computer storage medium(such as a disk, tape, physical hard drive, or virtual hard drive suchas in a storage access network). If the designer does not fabricatechips or the photolithographic masks used to fabricate chips, thedesigner may transmit the resulting design by physical means (e.g., byproviding a copy of the storage medium storing the design) orelectronically (e.g., through the Internet) to such entities, directlyor indirectly. The stored design is then converted into the appropriateformat (e.g., GDSII) for the fabrication of photolithographic masks,which typically include multiple copies of the chip design in questionthat are to be formed on a wafer. The photolithographic masks areutilized to define areas of the wafer (and/or the layers thereon) to beetched or otherwise processed.

Methods as described herein may be used in the fabrication of integratedcircuit chips. The resulting integrated circuit chips can be distributedby the fabricator in raw wafer form (that is, as a single wafer that hasmultiple unpackaged chips), as a bare die, or in a packaged form. In thelatter case, the chip is mounted in a single chip package (such as aplastic carrier, with leads that are affixed to a motherboard or otherhigher level carrier) or in a multichip package (such as a ceramiccarrier that has either or both surface interconnections or buriedinterconnections). In any case, the chip is then integrated with otherchips, discrete circuit elements, and/or other signal processing devicesas part of either (a) an intermediate product, such as a motherboard, or(b) an end product. The end product can be any product that includesintegrated circuit chips, ranging from toys and other low-endapplications to advanced computer products having a display, a keyboardor other input device, and a central processor.

Referring now to the drawings in which like numerals represent the sameor similar elements and initially to FIG. 1, a varactor 100 is formed onor in a substrate 102 with a graphene sheet 108 on a dielectric 106between two electrodes 104 and 110. A total capacitance between the twoelectrodes 104, 110 is a series combination of a dielectric capacitance(C_(OX)) of dielectric 106 and a quantum capacitance from graphene(C_(Q)). A voltage difference between the two electrodes 104, 110produces an electric field across the dielectric 106 and the graphenesheet 108. While C_(OX) is constant, C_(Q) exhibits a strong dependenceon the electric field. Hence, the total capacitance depends on thevoltage between the electrodes 104 and 110 and a variable capacitor(varactor 100) is obtained.

It should be understood that the graphene sheet 108 acts, not as atransistor channel but as a capacitor dielectric with a quantumsensitivity to applied voltage. The quantum capacitance is not presentin conventional semiconductors such as silicon. Moreover, its value canchange by one or two orders of magnitude with moderate voltage controlvariations (e.g., 1-2 V). For many circuit applications, it is desiredthat a varactor features a largest possible variation between themaximum and minimum capacitance. For this purpose, in the graphenevaractor 100, C_(Q) should dominate the total varactor capacitance. Toaccomplish this, C_(OX) should be as large as possible and therefore ahigh-k dielectric for dielectric 106 is preferably employed. Thevaractor 100 provides a higher quality dielectric deposition achievableby its embedded electrode structure. Moreover, both electrodes 104 and110 completely cover the graphene 108, minimizing contact resistance andimproving the varactor's quality factor.

FIGS. 2-7 show an illustrative method for fabricating the varactor 100of FIG. 1. Referring to FIG. 2, substrate 102 is provided or formed.Substrate 102 may be formed on another layer or material. In oneembodiment, substrate 102 may be formed on a semiconductor material,such as a bulk silicon, GaAs, Ge, SiGe or other substrate, e.g.,semiconductor on insulator (SOI). Substrate 102 includes a dielectricmaterial, such as silicon dioxide, silicon carbide or other inorganic ororganic dielectric material.

Referring to FIG. 3, a trench 103 is etched into the substrate 102. Thismay include forming a mask and etching the trench 103 by a known etchingmethod, such as, e.g., a reactive ion etch (RIE) method. The mask may beformed by a lithographic process. The trench 103 is formed to enable theformation of an embedded electrode as will be described with respect toFIG. 4.

Referring to FIG. 4, a conductive material is deposited and processed toform an electrode 104. The conductive material may include a depositedmetal, such as copper, aluminum, gold, silver, tungsten, etc. Otherconductive material such as doped polysilicon, organic conductors, etc.may also be employed. The conductive material is likely uniformlydeposited to fill the trench 103 and cover portions of the substrate102. To confine the conductive material to the trench 103, a planarizingprocess such as a chemical mechanical polish (CMP) or an etching processmay be employed to form the electrode 104 at or below a surface of thesubstrate 102.

Referring to FIG. 5, dielectric 106 is formed over the substrate 102 andthe electrode 104. While the dielectric 106 may include a silicondioxide, silicon carbide, etc., since Cox is desired to be maximized toobtain optimum performance of the varactor 100, a dielectric with a highdielectric constant (e.g., higher than 3.9) is preferred for dielectric106. Dielectric 106 may include, e.g., hafnium dioxide, hafniumoxynitride, silicon oxynitride, hafnium silicate, zirconium silicate,zirconium dioxide, etc. The high-K dielectric for dielectric 106 may bedeposited by, e.g., an atomic layer deposition process. Since electrode104 is embedded and has a surface even with substrate 102, high-Kdielectric deposition is supported, which is generally very difficult touniformly form on graphene due to its inert surface.

Referring to FIG. 6, dielectric 106 is suited for the formation of agraphene material 108 thereon. Graphene material 108 may include achemical vapor deposited (CVD) layer, an epitaxially grown layer, asolution based deposited layer (dipping), a mechanically exfoliatedlayer (transferred layer), etc. The process by which graphene isdeposited on the dielectric 106 may vary with the material of thedielectric 106, expense and/or other factors. For example, amechanically exfoliated graphene or CVD grown graphene can betransferred on an oxide. This embodiment provides a flat and evensurface on which to form the graphene material 108. Since the dielectric106 is formed flat, applying the graphene material 108 by a dippingprocess or a transfer process is enabled. These processes greatlysimplify workflow and reduce cost.

The graphene material 108 may be roughly formed in terms of coverage onthe dielectric 106 since the graphene may be shaped in a later process.The graphene material 108 may be formed with between about 1 to about 10or more graphene layers. While additional layers may also be useful, asingle layer of graphene material 108 is preferred. The graphenematerial 108 may be patterned to adjust its shape. A lithographic maskmay be formed on the graphene 108, and the graphene material 108 ispatterned, for example, by a lithographic development process and etchedto form a shaped graphene material 108. The mask (not shown) is thenremoved to expose the shaped graphene material 108. The graphenematerial 108 may be formed in any shape. The graphene material 108 inundesirable areas is preferably removed by, e.g., an oxygen plasma baseddry etch. Other etching or patterning processes may also be employed.

Referring to FIG. 7, a top electrode 110 is formed on the graphenematerial 108. Electrode 110 preferably includes a same material aselectrode 104 although other materials may be selected. Electrode 110may be formed by employing a lift-off process. The lift-off process mayinclude forming a sacrificial layer, which is deposited and an inversepattern is created (e.g., using a photoresist, which is exposed anddeveloped). The inverse pattern includes holes where conductive materialfor electrode 110 should remain on the graphene material 108. Forexample, the photoresist is removed in the areas where the electrode 110is to be located. Conductive material is deposited over the photoresistand exposed graphene material 108. The rest of the sacrificial material(photoresist) is washed out together with parts of the conductivematerial covering the photoresist. Only the material that was in theholes and having direct contact with the graphene material 108 remains.Other processes such as, e.g., masking and etching a conductive layermay also be employed to form electrode 110. Additional processing maycontinue, e.g., forming connections to the plate electrodes 104 and 110of the plate varactor 100, forming additional interlevel dielectrics,etc.

Referring to FIG. 8, a varactor 200 is formed on a substrate 202 using agraphene sheet 204 covered by a dielectric 208 and having two electrodes206 and 210. As before, the total capacitance between the two electrodes206, 210 is a series combination of a dielectric capacitance (C_(OX))and a quantum capacitance from graphene (C_(Q)). A voltage differencebetween the two electrodes 206, 210 produces an electric field acrossthe dielectric 208 and the graphene sheet 204. While C_(OX) is constant,C_(Q) exhibits a strong dependence on the electric field. Hence, thetotal capacitance depends on the voltage between the electrodes 206 and210 and a variable capacitor (varactor 200) is obtained.

It should be understood that the graphene sheet 204 acts, not as atransistor channel but as a capacitor dielectric with a quantumsensitivity to applied voltage. The quantum capacitance can change byone or two orders of magnitude with moderate voltage control variations(e.g., 1-2 V). In graphene varactor 200, C_(Q) should dominate the totalvaractor capacitance by making C_(OX) as large as possible. Therefore, ahigh-k dielectric for dielectric 208 is preferably employed. Reliefzones 216 are optionally provided and will be explained in greaterdetail below.

FIGS. 9-14 show an illustrative method for fabricating the varactor 200of FIG. 8. Referring to FIG. 9, substrate 202 is provided or formed.Substrate 202 may be formed on another layer or material. In oneembodiment, substrate 202 may be formed on a semiconductor material,such as a bulk silicon, GaAs, Ge, SiGe or other substrate, e.g.,semiconductor on insulator (SOI). Substrate 202 includes a dielectricmaterial, such as silicon dioxide, silicon carbide or other inorganic ororganic dielectric material.

Referring to FIG. 10, a graphene material 204 is formed or transferredto a surface of the substrate 202. Graphene material 204 may include achemical vapor deposited (CVD) layer, an epitaxially grown layer, asolution based deposited layer (dipping), a mechanically exfoliatedlayer (transferred layer), etc. The process by which graphene isdeposited on the substrate 202 may vary with the material of thesubstrate 202, expense and/or other factors. For example, a mechanicallyexfoliated graphene or CVD grown graphene can be transferred on anoxide. Alternatively, graphene may be provided on a silicon carbidematerial. The substrate 202 provides a flat and even surface on which toform the graphene material 204. Since the substrate 202 is flat,applying the graphene material 204 by a dipping process or a transferprocess is enabled. The graphene material 204 may be roughly formed interms of coverage on the substrate 202 since the graphene may be shapedby a patterning process (e.g., lithography) (see FIG. 11). The graphenematerial 204 may be formed with between 1 to about 10 or more graphenelayers. While additional layers may also be useful, a single layer ofgraphene material 204 is preferred.

Referring to FIG. 11, the graphene material 204 may be patterned toadjust its shape. A lithographic mask (e.g., a photoresist) 205 may beformed on the graphene material 204, patterned, for example, by alithographic development process. The graphene is etched to form ashaped graphene material 204. The mask 205 is then removed to expose theshaped graphene material 204. The graphene material 204 may be formed inany shape. The graphene material 204 in undesirable areas is preferablyremoved by, e.g., an oxygen plasma based dry etch. Other etching orpatterning processes may also be employed.

Referring to FIG. 12, a conductive material is deposited and processedto form an electrode 206. The conductive material may include adeposited metal, such as copper, aluminum, gold, silver, tungsten, etc.Other conductive material such as doped polysilicon, organic conductors,etc. may also be employed. The conductive material may be uniformlydeposited over the graphene 204 and portions of the substrate 202. Alithography process may be employed to etch away portions of theconductive layer to expose a portion of the graphene 204. The electrode206 is formed over the edges or periphery of the graphene 204 withsufficient coverage area to provide adequate resistance when thevaractor 200 is completed.

In another embodiment, a lift-off process may be employed to formelectrode 206. A sacrificial material (not shown), such as a photoresistmay be formed and patterned to occupy a portion of the graphene 204surface. The conductive material is deposited over the substrate,exposed portions of the graphene 204 and the sacrificial material. Then,the lift-off process breaks down the sacrificial material to leave thestructure depicted in FIG. 12. Other methods may be employed to form theelectrode 206.

Referring to FIG. 13, a dielectric 208 is formed over the graphene 204,the electrode 206 and the substrate 202. While the dielectric 208 mayinclude a silicon dioxide, silicon carbide, etc., since Cox is desiredto be maximized to obtain optimum performance of the varactor 200, adielectric with a high dielectric constant (e.g., higher than 3.9) ispreferred for dielectric 208. Dielectric 208 may include, e.g., hafniumdioxide, hafnium oxynitride, silicon oxynitride, hafnium silicate,zirconium silicate, zirconium dioxide, etc. The high-K dielectric fordielectric 208 may be deposited by, e.g., an atomic layer depositionprocess. The dielectric 208 is patterned to remove the dielectric 208from larger portion of the electrode 206. The dielectric 208 may bepatterned using a lithography process to form a mask and etch with a wetetch or RIE process.

Referring to FIG. 14, a top electrode 210 is formed on the dielectric208. Electrode 210 preferably includes a same material as electrode 206although other materials may be selected. Electrode 210 may be formed byemploying a lift-off process. The lift-off process may include forming asacrificial layer, which is deposited and an inverse pattern is created(e.g., using a photoresist, which is exposed and developed). The inversepattern includes holes where conductive material for electrode 210should remain on the dielectric material 208. For example, thephotoresist is removed in the areas where the electrode 210 is to belocated. Conductive material is deposited over the photoresist, exposeddielectric 208, electrode 206 and other areas. The rest of thesacrificial material (photoresist) is washed out together with parts ofthe conductive material covering the photoresist. Only the material thatwas in the holes and having direct contact with the dielectric 208remains. Other processes such as, e.g., masking and etching a conductivelayer may also be employed to form electrode 210. The formation processfor electrode 210 may provide relief zones 216 at a periphery of theelectrode 210 such that contact with the dielectric 208 on verticalsurfaces is limited. This assists in maintaining the plate capacitorarea without including the vertical surfaces. In some embodiments, theelectrode 210 may fill the relief zones 216, as shown in FIG. 18.Additional processing may continue, e.g., forming connections to theplate electrodes 206 and 210 of the plate varactor 200, formingadditional interlevel dielectrics, etc.

Referring to FIG. 15, shows three simulation plots 301, 303 and 305 ofcapacitance (in fF/micron²) versus V_(TG) (in volts) applied toelectrodes separated by a dielectric layer and graphene for varactors inaccordance with the present principles. The three plots 301, 303 and 305have different equivalent oxide thicknesses (EOT). These include EOT=0.1nm for plot 301, EOT=0.5 nm for plot 303, and EOT=1.0 nm for plot 305.

The plots 301, 303, 305 show total capacitance 302 (as:C_(OX)*C_(Q)/(C_(OX)+C_(Q))) and total capacitance 304 (as:d(Q_(net))/dV_(TG), which is the derivative of the net charge withrespect to V_(TG)). The total capacitances 302 and 304 coincide in theplots 301, 303, 305. The plots 301, 303, 305 further show C_(ox) 308 andC_(Q) 306.

The plots 301, 303, 305 show a large (>10) capacitance tunabilitythrough graphene quantum capacitance (C_(Q)). In fact, greater than 10times tunability (e.g., C_(max)/C_(min)) is provided with V_(TG)<1 V.Graphene dielectric capacitors may have many applications including inRF circuits (without the need for Si hybrid technology) and otherdevices in accordance with the present principles. The presentprinciples provide easy process flow, higher tunability and betterperformance than conventional counterparts. In particularly usefulembodiments, the varactors in accordance with the present principles maybe used in or with high performance RF/mmWave graphene ICs.

Referring to FIG. 16, a method for fabricating a plate varactor isdescribed in accordance with particularly useful embodiments. In block402, a trench is formed in a dielectric substrate. In block 404, anembedded electrode is formed in the trench of the substrate. This mayinclude depositing a conductive material followed by a planarizationprocess to complete the embedded electrode. In block 406, a capacitordielectric layer is formed over the first electrode. The dielectriclayer preferably includes a high dielectric constant material (high-Kdielectric). High-K materials are more easily formed over a planarizedor flat surface. In block 408, a layer of graphene is provided over thedielectric layer to contribute a quantum capacitance component to thedielectric layer. The layer of graphene may include providing a singlelayer of graphene. The layer of graphene may be formed by dip coating,transferring, depositing, etc. The layer of graphene preferably isformed to the same extents as the embedded electrode. It should be notedthat the dielectric layer and the layer of graphene may be formed inreverse order with the dielectric layer being formed on the graphenelayer.

In block 410, an upper electrode is formed on the layer of graphene.This may include patterning the upper electrode to cover lateral extentsof the layer of graphene. In this way, the graphene layer is completelysandwiched by the embedded and upper electrodes. In block 412, thequantum capacitance component of the layer of graphene may be tuned byaltering an applied voltage to one of the embedded electrode and theupper electrode. In block 414, the tuning may include a change incapacitance of greater than a factor of ten for a one volt change in theapplied voltage to either the embedded electrode or the upper electrode.

Referring to FIG. 17, a method for fabricating a plate varactor isdescribed in accordance with other useful embodiments. In block 502, alayer of graphene is formed over a portion of a surface of a dielectricsubstrate. The graphene may include a single layer. In block 504, afirst electrode is formed contacting edges of a periphery of the layerof graphene and over portions of the surface of the substrate. Theelectrode may be formed using lithography and etching processes,lift-off processes, etc. In block 506, a dielectric layer is formed overthe layer of graphene and over a part of the first electrode contactingthe layer graphene. The dielectric layer and the layer of grapheneprovide a capacitor dielectric wherein the layer of graphene contributesa quantum capacitance component to the dielectric layer. The dielectriclayer preferably includes a high-K dielectric. The high-K dielectric isdeposited and patterned over the first electrode. In block 508, an upperelectrode is formed on the dielectric layer. In block 510, relief zonesmay be provided adjacent to the upper electrode to prevent verticalsides of the upper electrode from contacting the dielectric layer. Inblock 512, the quantum capacitance component of the layer of graphenemay be tuned by altering an applied voltage to one of the firstelectrode and the upper electrode. In block 514, the tuning may includea change in capacitance of greater than a factor of ten for a one voltchange in the applied voltage to either the first electrode or the upperelectrode.

Having described preferred embodiments for quantum capacitance graphenevaractors and fabrication methods (which are intended to be illustrativeand not limiting), it is noted that modifications and variations can bemade by persons skilled in the art in light of the above teachings. Itis therefore to be understood that changes may be made in the particularembodiments disclosed which are within the scope of the invention asoutlined by the appended claims. Having thus described aspects of theinvention, with the details and particularity required by the patentlaws, what is claimed and desired protected by Letters Patent is setforth in the appended claims.

What is claimed is:
 1. A voltage dependent capacitance device,comprising: a plate varactor, wherein the plate varactor includes: adielectric substrate; a layer of graphene formed directly on a portionof a top surface of the dielectric substrate; a first electrode formedcontacting edges of an entire periphery of the layer of graphene andover portions of the top surface of the dielectric substrate; adielectric layer formed on top of the layer of graphene and on top of apart of the first electrode contacting the layer of graphene, whereinthe dielectric layer includes a vertical surface, the dielectric layerand the layer of graphene providing a capacitor dielectric, wherein thelayer of graphene contributes a voltage dependent quantum capacitancecomponent to the dielectric layer; and an upper electrode formed on thedielectric layer, wherein the vertical surface of the dielectric layersurrounds the entire periphery of the upper electrode, and wherein theupper electrode is in contact with the vertical surface of thedielectric layer, and wherein the first electrode comprises a two-leveltop surface, and wherein a level change in the two-level top surfaceoccurs directly over a step up formed by the periphery of the layer ofgraphene with respect to the top surface of the dielectric substrate. 2.The device as recited in claim 1, wherein the capacitor dielectricincludes a high-K dielectric.
 3. The device as recited in claim 1,wherein the layer of graphene includes a single layer of graphene. 4.The device as recited in claim 1, wherein the quantum capacitancecomponent of the layer of graphene provides tunability of capacitancebased on an applied voltage to one of the first electrode and the upperelectrode.
 5. The device as recited in claim 4, wherein the tunabilityis greater than a factor of ten for a one volt change in the appliedvoltage to one of the first electrode and the upper electrode from zerovolts.
 6. The device as recited in claim 1, wherein the first electrodecomprises first portions and second portions, the first portions beingformed on top of the surface of the substrate and contacting a surfaceof the layer of graphene and the second portions being formed directlyon the surface of the substrate.
 7. The device as recited in claim 1,wherein the dielectric layer is formed on top of the layer of grapheneand on top of the part of the first electrode contacting the layer ofgraphene, so as to be vertically above the layer of graphene andvertically above the part of the first electrode contacting the layer ofgraphene.
 8. The device as recited in claim 1, wherein the dielectriclayer is formed on top of only a portion of a top surface of the firstelectrode.
 9. The device as recited in claim 8, wherein the dielectriclayer covers only a portion of a top level and none of a bottom level ofthe two-level top surface of the first electrode.
 10. A voltagedependent capacitance device, comprising: a plate varactor, wherein theplate varactor includes: a dielectric substrate; a layer of grapheneformed directly on a portion of a surface of the dielectric substrate; afirst electrode formed contacting edges of a periphery of the layer ofgraphene and over portions of the surface of the dielectric substrate,wherein the first electrode comprises a two-level top surface, wherein alevel change in the two-level top surface occurs directly over a step upformed by the edges of the periphery of the layer of graphene withrespect to a top surface of the dielectric substrate; a dielectric layerformed on top of the layer of graphene and on top of a part of the firstelectrode contacting the layer of graphene, wherein the dielectric layerincludes a vertical surface, the dielectric layer and the layer ofgraphene providing a capacitor dielectric, wherein the layer of graphenecontributes a voltage dependent quantum capacitance component to thedielectric layer; and an upper electrode formed on the dielectric layer,wherein the upper electrode is in contact with the vertical surface ofthe dielectric layer, wherein the quantum capacitance component of thelayer of graphene provides a tunability of capacitance based on anapplied voltage to one of the first electrode and the upper electrode,wherein the tunability is greater than a factor of ten for a one voltchange in the applied voltage to one of the first electrode and theupper electrode from zero volts.